Certain biometric measurements are subject to noise which makes it very difficult to provide a proper analysis of the sensor signals. In particular, pulse oximetry measurements are noise sensitive. Pulse oximetry uses a pulse oximeter which is a non-invasive medical device that monitors the oxygen saturation of a patient's blood and heart rate.
Referring to FIGS. 5 and 6, depicted are schematics of a general overview block diagram and a more detailed block diagram of a prior technology pulse oximetry measurement system. A pulse oximeter monitors the oxygen saturation (SpO2) of a human's blood based on the red light (600-750 nm wavelength) and infrared light (850-1000 nm wavelength) absorption characteristics of oxygenated hemoglobin (HbO2) and deoxygenated hemoglobin (Hb). The pulse oximeter flashes red and infrared lights, e.g., light emitting diodes (LEDs), alternately through a finger to a photodiode. HbO2 absorbs more infrared light and allows more red light to pass through. On the other hand, Hb absorbs more red light and allows more infrared light to pass through. The photodiode receives the non-absorbed light from each LED. The light intensity measurements for each red and infrared light source must be taken at different times and ambient light (background light noise) will affect these measurements. Flickering lights (e.g., fluorescent lights) as a noise source are difficult to eliminate.
Operation of the circuits shown in FIGS. 5, 6 and 7 are more fully explained in Application Note AN1525, “Pulse Oximeter Design Using Microchip's Analog Devices and dsPIC® Digital Signal Controllers (DSCs)” by Zhang Feng, published 2013 by Microchip Technology Inc., available at www.microchip.com, and is hereby incorporated by reference herein for all purposes. FIG. 7 shows exemplary waveforms displaying the pulse signals from the pulse oximeter systems of FIGS. 5 and 6.